# std::uninitialized_copy_n

From cppreference.com
< cpp‎ | memory

Defined in header `<memory>` |  |
template< class InputIt, class Size, class NoThrowForwardIt > NoThrowForwardIt uninitialized_copy_n( InputIt first, Size count, NoThrowForwardIt d_first ); | (1) | (since C++11) (constexpr since C++26)
template< class ExecutionPolicy, class ForwardIt, class Size, class NoThrowForwardIt > NoThrowForwardIt uninitialized_copy_n( ExecutionPolicy&& policy, ForwardIt first, Size count, NoThrowForwardIt d_first ); | (2) | (since C++17)

1) Copies count elements from a range beginning at first to an uninitialized memory area beginning at d_first as if by

for (; count > 0; ++d_first, (void) ++first, --count)
`` ::new (`voidify`(*d_first))
`` typename std::iterator_traits<NoThrowForwardIt>::value_type(*first);

If an exception is thrown during the initialization, the objects already constructed are destroyed in an unspecified order.
2) Same as (1), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If d_first`+` `[`​0​`,` count`)` overlaps with first`+` `[`​0​`,` count`)`, the behavior is undefined. | (since C++20)

### Parameters

first | - | the beginning of the range of the elements to copy
count | - | the number of elements to copy
d_first | - | the beginning of the destination range
policy | - | the execution policy to use
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `NoThrowForwardIt` must meet the requirements of LegacyForwardIterator.
- No increment, assignment, comparison, or indirection through valid instances of `NoThrowForwardIt` may throw exceptions.

### Return value

Iterator to the element past the last element copied.

### Complexity

Linear in count.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_raw_memory_algorithms` | `202411L` | (C++26) | constexpr for specialized memory algorithms, (1)

### Possible implementation

    template<class InputIt, class Size, class NoThrowForwardIt>
    constexpr NoThrowForwardIt uninitialized_copy_n(InputIt first, Size count,
                                                    NoThrowForwardIt d_first)
    {
        using T = typename std::iterator_traits<NoThrowForwardIt>::value_type;
        NoThrowForwardIt current = d_first;
        try
        {
            for (; count > 0; ++first, (void) ++current, --count)
                ::new (static_cast<void*>(std::addressof(*current))) T(*first);
        }
        catch (...)
        {
            for (; d_first != current; ++d_first)
                d_first->~T();
            throw;
        }
        return current;
    }

### Example

Run this code

    #include <algorithm>
    #include <iostream>
    #include <memory>
    #include <string>
    #include <tuple>
    #include <vector>

    int main()
    {
        std::vector<std::string> v = {"This", "is", "an", "example"};

        std::string* p;
        std::size_t sz;
        std::tie(p, sz) = std::get_temporary_buffer<std::string>(v.size());
        sz = std::min(sz, v.size());

        std::uninitialized_copy_n(v.begin(), sz, p);

        for (std::string* i = p; i != p + sz; ++i)
        {
            std::cout << *i << ' ';
            i->~basic_string<char>();
        }
        std::cout << '\n';

        std::return_temporary_buffer(p);
    }

Possible output:

    This is an example

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2133 | C++98 | the effect description used a for loop with the iteration expression ++d_first, ++first, --count, which results in argument-dependent lookups of operator, | discards the value of one operand to disable those ADLs
LWG 2433 | C++11 | this algorithm might be hijacked by overloaded operator& | uses std::addressof
LWG 3870 | C++20 | this algorithm might create objects on a const storage | kept disallowed

### See also

uninitialized_copy | copies a range of objects to an uninitialized area of memory (function template)
ranges::uninitialized_copy_n (C++20) | copies a number of objects to an uninitialized area of memory (algorithm function object)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/memory/uninitialized_copy_n&oldid=179703"